1. Field of the Invention
The present invention relates generally to microelectronic devices and, more particularly, to the housing of microelectronic devices.
2. Description of the Related Art
Microelectronic devices are typically housed to provide protection from environmental factors (e.g., moisture, dust and other contaminants) and yet provide operational access to the devices. A variety of microelectronic-device housings have been disclosed (e.g., see U.S. Pat. Nos. 5,021,864, 5,122,858, 5,389,577, 5,929,514 and 5,969,414) and an exemplary housing is illustrated by the conventional microelectronic-device assembly 20 of FIGS. 1 and 2.
The assembly 20 includes a microelectronic device 22, bonding wires 24 and a lead frame 26 that defines a tie bar 28 that is coupled to a paddle 30 by tie-bar supports 31. The lead frame also defines a plurality of elongate leads 32 that each have an inner end 33 directed towards the microelectronic device 22 and an outer end 34 that couples to the tie bar 28.
The microelectronic device 22 has a back face 35 and a front or circuit face 36 that includes a plurality of bonding pads 38. The bonding pads are generally located proximate to the perimeter of the circuit face 36 and are connected to various microelements within the microelectronic device 22 to thus provide operational access to the microelectronic device.
The back face 35 has a back-face area and the front circuit face 36 has a circuit-face area that typically equals the back-face area. In contrast, the paddle 30 has a paddle area that generally exceeds the back-face area to thereby facilitate carrying the back face 35 on the paddle 30 (accordingly, portions of the paddle 30 are typically visible all about the perimeter of the microelectronic device 22). The back face 34 is generally secured to the paddle with an adhesive 40 (e.g., an epoxy adhesive or tape) or with solder.
Each of the bonding wires 24 is coupled to a respective one of the bonding pads 38 and a respective one of the inner lead ends 33. The bonding wires permit the bonding pads to be accessed from the outer lead ends 34.
As shown in FIG. 2, a plastic overmold 48 is typically formed to encapsulate the microelectronic device 22, the bonding wires 24, the paddle 30 and the inner ends 33 of the leads 32. After this encapsulation, the tie bar (28 in FIG. 1) and its tie-bar supports (31 in FIG. 1) are removed so that only the outer ends 34 extend from the overmold 48 for operational access of the microelectronic device 22.
The lead frame 26 is typically formed from a conductive metal (e.g., a nickel-iron or a copper-based alloy), the bonding wires are typically aluminum or gold, the overmold 48 is generally a thermosetting material (e.g., epoxy, silicone, or urethane) and exemplary dimensions of the bonding pads are 100xc3x97100 xcexcm.
Although it may take on other forms, the microelectronic device 22 is typically a semiconductor die that contains a large number of microelements organized in an integrated circuit and, accordingly, the back face 35 is generally a semiconductor substrate. The integrated-circuit microelements are often microelectronic elements (e.g., transistors, resistors, capacitors and interconnection traces) but they may also be micromachined mechanical elements that include movable members (e.g., the vibrating members of a gyroscope or the acceleration-responsive mass of an accelerometer).
Microelectronic elements may also include movable members (e.g., the arm of a microswitch or microrelay) and, in addition, the performance of some microelectronic elements (e.g., thin film resistors) may be sensitive to pressure. Operation of other exemplary microelements may be degraded by outgassing of constituents from various assembly parts (e.g., the overmold).
When these sensitive microelements are positioned proximate to an area of the front circuit face 36, it is desirable to exclude extraneous elements (i.e., elements that restrict or degrade operation) from this sensitive area. Although a thin cover coat (e.g., a polyimide) is conventionally applied to the front circuit face 36 before the microelectronic device 22 is encapsulated in the overmold 48, such coatings fail to provide the desired exclusion.
The present invention is directed to microelectronic-device assemblies and methods that enhance operation of microelectronic devices because they exclude extraneous elements from sensitive device areas.
The invention is particularly suited for a device that has a sensitive area on a device face that carries a plurality of bonding pads. In an embodiment, a paddle is provided that has an area less than that of the device face and it is spaced from the face and positioned to cover the sensitive area and expose the pads. A plastic ring is arranged to surround the sensitive area and abut the face and the paddle.
A lead frame preferably forms the paddle and a plurality of elongate leads. Each of the leads has an inner end and an outer end and each of a plurality of bonding wires is coupled between a respective one of the pads and a respective one of the inner ends.
An overmold is arranged to encapsulate the die, the paddle, the ring and the inner lead ends and expose the outer lead ends to provide operational access to the microelectronic device. A void is thus defined within the ring and between the device face and the paddle and, accordingly, extraneous elements are excluded from this void. When the plastic ring includes an electrically-conductive filler, it and the paddle provide an electrical and magnetic shield for the device face.
The novel features of the invention are set forth with particularity in the appended claims. The invention will be best understood from the following description when read in conjunction with the accompanying drawings.